This invention is a further improvement on the improved magnetic field sensing system already being described and claimed in the above-identified United States patent application Ser. No. 229,396, filed in the names of Thaddeus Schroeder and Bruno P. B. Lequesne and entitled, "Position Sensor."
The need for accurately and easily sensing position, speed or acceleration is growing, particularly in the automotive field. Anti-lock braking systems, traction control systems, electric power steering, four-wheel steering and throttle control are examples of functions that can use such sensing. Such applications not only require accuracy and precision, but frequently involve severe environments. Cost of such systems is an important factor, too.
For such applications, it is desirable to have a position sensor (speed and acceleration can be derived from a position signal) that is rugged and reliable, small and inexpensive, capable of low (including zero) speed sensing and relatively immune to electromagnetic field interference from the other systems used in an automobile.
A well-known form of position sensor is a semiconductor magnetoresistive sensor. Such a sensor comprises a magnetic circuit that includes two basic parts. One of these parts, typically kept stationary, includes a semiconductive sensing element that is sensitive to the magnetic flux density passing through its surface, and further includes a permanent magnet for creating a reference flux. The other of the two parts, termed the exciter, includes a high magnetic permeability element with a series of teeth that moves with relation to the stationary element for changing the reluctance of the magnetic circuit and for causing the magnetic flux through the sensing element to vary in a fashion corresponding to the position of the teeth.
Such a sensor is sensitive to the magnetic flux density rather than to the rate of flux density change and so it does not have a lower speed limit. This also makes it less sensitive to E.M.I. Moreover, its response is predictably related to the distribution of flux density over the surface of the sensing element.
Typically, the stationary part includes a magnetoresistive element, including a semiconductive element whose resistance varies with the magnetic flux density passing through it in controllable fashion so that an electrical output signal can be derived. Moreover, when this magnetoresistor is produced from a high electron mobility semiconductor, such as compound semiconductors like indium antimonide or indium arsenide, a large electrical output signal can be available. If the output signal is sufficiently large, there is the possibility of providing an output signal that requires little or no further amplification, a factor of considerable advantage. It is desirable to have a position sensor of high sensitivity so that a large electrical output signal can be produced efficiently and of easy manufacture so that it can be made reliably and at low cost.
The magnitude of the flux variations in the sensing element for a given change in position of the exciter is an important factor in determining the sensitivity of the sensor. Accordingly, a variety of designs have been attempted hitherto to maximize the change in the flux density through the sensor in response to a given change in exciter position. Typically, these attempts involved including a flux guide for the permanent magnet included in the stationary part of the magnetic circuit to provide a return path for the magnetic field of the magnet. Additionally, sometimes a field concentrator of commensurate size has been provided contiguous to the magnetoresistive element to concentrate flux through the magnetoresistive element.
However, for example, such techniques have typically produced magnetic circuit sensitivities no higher than about five percent for a typical exciter design having a three millimeter tooth pitch and one millimeter gap, where the sensitivity is defined as the difference between the maximum and minimum flux densities sensed divided by the mean flux density sensed (half the sum of the maximum and minimum flux densities sensed).
Previously referred to U.S. patent applications Ser. No. 289,634 and Ser. No. 289,646 describe the fabrication and properties of a new type of magnetoresistor thin film element. Application Ser. No. 289,634 details the process of growing a low to moderate conductivity thin film of indium arsenide (InAs), a narrow-gap semiconductor, on a semi-insulating indium phosphide (InP) substrate, and shows that this device has a rather large sensitivity of electrical resistance to magnetic field. Application Ser. No. 289,646 outlines various methods of enhancing the sensitivity of the device on the basis of the existence of a thin surface layer (known as an accumulation or inversion layer) of high density, high mobility electrons. Such electron accumulation or strong inversion layers can be induced in a variety of semiconductor thin films materials. While the devices described therein could be used in a wide variety of magnetic field sensing applications without significant further development, the application of these magnetoresistors as position sensors in more stringent operating conditions (such as those which exist in an automobile) requires interfacing the magnetoresistor with a suitable sensing system.
We have recognized that the Schroeder and Lequesne (USSN 229,396) type of magnetic circuit is so effective in concentrating the magnetic field that lesser sensitive magnetoresistors may still work well enough to be useful at some applications. In addition, we have recognized that some of the less sensitive magnetoresistor materials are magnetically sensitive at higher temperatures. We have also recognized that the improved magnetoresistor concepts of USSN 289,634 and USSN 289,646 provide enhancement to lesser magnetically sensitive materials. We have thus recognized that the combination of all these concepts could provide especially striking benefits. This patent application specifically describes and claims that combination.
There are several reasons why the improved magnetoresistors described in USSN 289,634 and 289,646 would be especially desirable for use in such a sensing system. The reasons will not be mentioned in order of importance. First, extreme compactness of these sensors make their use ideal in any sensing location, regardless of the space limitations. Secondly, their improved sensitivity to magnetic field affords the designer a large amount of freedom in the placement of the sensor with respect to the exciter wheel. This means that the air gap between exciter and sensor can be larger than for a less sensitive device without any diminution in magnitude of the electrical signal. This could prove to be important in applications where vibration and thermal expansion problems limit the degree of proximity of the sensor to the exciter wheel. Also, the outstanding temperature stability of the sensitivity of the improved magnetoresistors will allow their application in extreme temperature environments, such as automotive anti-lock braking systems, in which temperatures can range from -50.degree. C. to +200.degree. C. Other applications may require operation at temperatures as high as +300.degree. C. We believe that the enhancement to system sensitivity afforded by the USSN 229,396 concepts and the enhancement to magnetoresistor sensitivity afforded by the USSN 289,634 and USSN 289,646 concepts, in combination, makes a wider group of semiconductor materials now available for use in magnetic field sensing. Materials that were previously considered as unacceptable now can be used, and will provide acceptable performance at much higher temperature. This expands the range of applications where such sensing is practical, and provides other benefits as well.
Accordingly, we think that the combination proposed in this patent application is especially attractive for automotive applications as part of linear or rotary position measurement systems. The sensitivity to magnetic field and high thermal stability of these sensors would be especially beneficial.